Multisection acoustic filter



Patented Get. 20, 1953 MULTISECTION ACOUSTIC FILTER Harry F. Olson, Princeton, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application April 29, 1947, Serial No. 744,548

3 Claims.

This invention relates to acoustic filters, and more particularly to an acoustic filter for use with various sound sources, especially sound reproducing instruments, for the purpose of limiting the high frequency response thereof.

It is well known that, in most radio receivers and phonographs of conventional design, considerable distortion and noise are introduced when the high frequency limit of reproduction extends beyond about 5000 cycles. To provide sound reproducing instruments which reproduce the entire audio range without noise or distortion is a very costly matter. Hence, in low cost instruments, it is desirable to reduce the response over 5000 cycles as much as possible. Furthermore, it

is desirable to make the cut-off at the high fre quency end as sharp as possible because a gradual cut-off which attenuates below 4000 cycles impairs the reproduction. If the cut-off is gradual above 4000 cycles, some noise and distortion will result. In other words, the best compromise between reproduction on the one hand and distortion and noise on the other is obtained with a sharp cut-off.

Aside from the foregoing, the view has frequently been expressed among those responsible for design of radio receivers and phonographs that, regardless of the excellence of such instruments, the public does not, in general, favor high frequencies in music, and there is some evidence to support this view. If such be the case, then it might well be desirable to limit the high frequency response of original sound sources such as orchestras, individual musical instruments, etc., as well as the response of sound reproducing instruments.

The primary object of my present invention is to provide an improved acoustic filter which will effectively eliminate or attenuate the high frequency response of any source of sound.

More particularly, it is an object of my present invention to provide an improved acoustic filter for attenuating high frequency sounds which is especially suitable for use with sound reproducing instruments, such as radio receivers and phonographs.

A further object of my present invention is to provide an mproved acoustic filter of the type set forth which can be made to provide sharp cut-off.

Another object of my present invention is to provide an improved acoustic filter as aforesaid which can be arranged in one or more stages to provide various degrees of attenuation.

Still another object of my present invention 4 is to provide an improved acoustic filter as above set forth which can be readily applied to existing sound reproducing instruments.

It is also an object of my present invention to provide an improved acoustic filter for the purpose set forth which is extremely simple in construction, highly efficient in use and inexpensive in cost.

In accordance with my present invention, the filter may consist of one or more stages or sections, each comprising a pair of parallel, perforated sheets or plates separated from each other a suitable distance and joined at their peripheries in any appropriate manner to enclose an air space therebetween. Two such plates constitute a single section filter. A two section filter consists of three such plates, one being common to each section; a three section filter consists of four such plates, etc. These filters may be placed in front of any sound source, such as the loudspeaker of a radio receiver, for example, or in proximity to one or more musical instruments or the like to reduce the high frequency response in each case. They may also be used in automobiles, buildings or other enclosures to attenuate external, high frequency sounds, thereby insulating the same against such noise. Filters of this sort provide a very sharp cut-off, the attenuation being as much as 20 db per section within a single octave.

The invention, both as to its organization and method of operation, as well as additional objects and advantages thereof, will be better understood from the following description of several embodiments thereof when read in connection with the accompanying drawing in which Figure 1 is a front elevation of one form of single stage or single section acoustic filter according to my present invention,

Figure 2 is a sectional view thereof taken along the plane of the line II--II of Figure 1.

Figure 3 is a view similar to Figure 2 of a twostage, or two section, filter,

Figure 4 is a similar view of a two section filter of somewhat different form,

Figure 5 is a fragmentary, sectional view of a sound reproducing instrument provided with a three section filter in accordance with my present invention,

Figure 6 is an electrical wiring diagram corresponding to the acoustical network of the system shown in Figure 5,

Figure 7 is a graph showing a family of response curves corresponding to a sound reproducing system such as illustrated in Figure 5 both without an acoustic filter and with acoustic filters of one or more stages according to my present invention.

Figure 8 is a fragmentary, sectional view showin how an acoustic filter according to my present invention may be applied to the window of a house or other building, and

Figure 9 is a diagrammatic view illustrating an auditorium in which an acoustic filter according to the present invention may be employed in connection with an original sound source.

Referring more particularly to the drawing,

wherein similar reference characters designate corresponding parts throughout, there is shown, in Figures 1 and 2 a low pass accoustic filter I comprised of a pair of perforated sheets or plates 3 and 5 which are arranged in parallel relation and are connected at their perimeters by a tubular frame or the like I to enclose an air space 9. Preferably, the member 7' is sealed to the plates 3 and 5 to provide an air seal. The openings or apertures I i in the plates 3 and 5 are all of the same diameter, the openings in the plate 3 being aligned with a like number of corresponding openings in the plate 5. By suitably dimensioning the openings H and the volume of the air space 9, a non-dissipative acoustic filter may be provided and be made to cut ofi at any desired high frequency. An attenuation of as much as db within a single octave higher than the cut-off frequency is possible with a single stage or single section filter such as above described.

In Figure 3, there is shown a two section filter. Here, the tubular member I is provided with a groove l3 in which a third plate 15, also perforated, is received. The plate i5 is disposed midway between and parallel to each of the plates 3 and 5 to provide two air spaces 9a and 9b of equal volume, the plate I5 being preferably also sealed to the member I. The plate 11 has the same number of openings as do each of the plates 3 and 5, the openings I! in the plate 15 being aligned with the corresponding openings II in the plates 3 and 5 but being of smaller diameter for a reason which will be apparent from the description hereinbelow with reference to Figure 5. The plates 3, 5 and I5 may be made of any suitable material, such as metal, wood, plastic, cardboard, glass, fabric, paper, or the like. One section of the filter consists of the plate 3, the plate 15, the intervening cavity or air space 9a and the surrounding portion of the tubular member I, and the other section of the filter consists of the plate IS, the plate 5, the intervening cavity 9b and the remaining portion of the member 1 which surrounds the latter. It will thus be seen that the intermediate plate i5 is common to both sections of the filter.

An acoustic filter which is made of two opposed, dish-like plates or the like 3a and 3b molded out of any suitable plastic material is illustrated in Figure 4. These may be heat-sealed together at their peripheral or marginal portions to provide a unitary structure. When only the members 3a and 3b are employed, a single stage, low pass filter results. If a plate l5a is interposed between the members 3a and 3b to provide two air spaces or cavities 9a and 9b, as shown in Figure 4, and the three plates heat-sealed together into a unitary structure, a two stage filter similar to that of Figure 3 results.

In Figure 5, I have shown a portion of a sound reproducing instrument which may be a radio receiver, a phonograph, or the like provided with a three section filter according to my present invention. This instrument includes a cabinet 2i the front wall of which is provided with an opening or the like 23 behind which is disposed a tubular supporting member 25 which is open at each end. A pair of perforated plates 21 and 29 corresponding, respectively, to the plates 3 and 5 of Fig. 3 and provided with aligned openings 28 of the same diameter are secured to the open ends of the tubular support 25. A pair of intermediate, perforated plates 3| and 33 provided with smaller openings 34 are carried by the tubular member 25 in suitably formed grooves 35 and 31. The plates 27, 29, 3| and 33 are all parallel to each other, the plates 3! and 33 being equally spaced from each other and from the plates 21 and 29 to provide air spaces 39a, 39b and 390 of equal volume. The openings 34 are aligned with each other and with the corresponding openings 28. The opening 23 in the cabinet may be covered in well known manner by a suitable ornamental grill or the like 4|. A loudspeaker diaphragm 43 having a flexible mounting 45 is mounted on the tubular member 25 behind the plate 21 and is so arranged that, upon vibration, it will generate sound waves into the tubular member 25. The four equally spaced, perforated plates 21, 3|, 33 and 39 and the intervening air spaces or cavities 39a, 39b and 390 constitute a three stage, low pass filter, the plate 3| being common to the first two sections of the filter, and the plate 33 being common to the second and third sections of the filter.

The acoustic network of the filter shown in Figure 5 may be represented by the wiring diagram of Figure 6 wherein p represents the pressure applied to the diaphragm 43 during vibration thereof,

MC represents the inertance of the moving system exemplified by the diaphragm 43, 7

1 Y represents the inertance of the air in each of the openings 28,

M1 represents the inertance of the air in each of the openings 34,

' MM represents the inertance of the air external to (in front of) the filter,

TAM represents the resistance of the air in f of the filter, rent CAC represents the capacitance of the flexible mounting 45 of the diaphragm 43,

Gav represents the capacitance of the volume of airdbetween the diaphragm 43 and the plate 21, an

CA1 represents the capacitance of the volume of air in each of the cavities 39a, 39b and 390.

It will be seen from Figure 6 that the filter of Figure 5 is constituted by three T-type, low pass filter sections. In the interest of efiiciency the end inertances 3 should be of about one-half the value of the intermediate inertances Ml. It is for this reason that the openings 28 are made larger than the openings 34. ii I The frequency at which the filter will cut off 15 determined by (1) the length of each of the openings 28 and 29 (that is, the thickness of each of the plates 21, 29, 3| and as), (2) the d ameters of the holes 28 and 34, and (3) the volume of each of the cavities 39a, 39b and 390 (that is, the distance between each adjacent pair of plates). The degree or amount of attenua- '3 tion will depend upon the number of stages or sections in the filter. In one installation employing a loudspeaker diaphragm 43 which was in diameter and employing a three section filter such as illustrated in Figure 5, a sharp cut off was obtained at 4500 cycles, as shown by the solid line curve A of Figure 7. To achieve this result, the four plates 21, 29, 3| and 33 were made of brass approximately 12" x 12 and thick and they were spaced apart from each other, the openings 28 were 0.150" in diameter, the

openings 34 were 0.090" in diameter, and all the openings 28 and 34 were spaced on centers. An attenuation of 60 db was obtained with this filter at an octave above the cut off frequency.

For a two stage filter of the same dimensions, an attenuation of 40 db would be obtained, as shown by the curve B. A single stage filter of the same dimensions would provide an attenuation of only db, as shown by the curve C of Figure 7. Without any filter at all, the response of the system in the high frequency region extended Well beyond 4500 cycles, as shown by the curve D.

Any desired out off frequency may be obtained with a filter such as described above. The cut on. frequency of an acoustic filter such as described above is given by the equation 1 1 1 r MCA Where The acoustic capacitance of a volume of enclosed air is given by the equation where V is the volume of the enclosure, p is the density of the air, and c is the velocity of sound.

The inertance of a round hole in a plate is given by the equation where p is the density of the air,

R is the radius of the hole, and

l is the length of the hole (that is, the thickness of the plate).

By suitable choice of constants or dimensions, which can be determined from Equations 1 to 3, a non-dissipative acoustic filter required to provide cut-off at any desired frequency can be obtained. Obviously, the greater the number of stages or sections in the filter, the sharper will be the cut-off.

In Figure 8, there is shown a fragmentary view of a building having a wall 51 provided with a window 53 in which a single stage filter 55, such as shown in Figs. 1 and 2, is inserted. Such a filter, if made of transparent material, such as Lucite or the like, will let in light and air but will keep out high frequency street noises which 6 are often very annoying. A suitably constructed filter may be used similarly in an automobile to keep out obnoxious, high frequency noises.

In Figure 9, there is illustrated, diagrammatically, an auditorium 6! having a plurality of rows of seats 63 for an audience, and a stage 65 where an orchestra or the like 66 may be located. It has been ascertained that many people find such high frequency sounds as are produced by violin bow scratch, and by flute, piccolo and similar wind noise quite annoying. To render the original music furnished by an orchestra or by one or more individual instruments more pleasing to such listeners, a filter 61 of one or more stages, such as described above, may be placed between the audience and the stage 65 behind a sound transparent, opaque screen 69. The acoustic filter 61 can be mounted on a suitable hoist 13, if desired, and raised behind 'a panel or the like II when its use is not desired.

From the foregoing description, it will be apparent to those skilled in the art that I have provided an improved acoustic filter of simple construction which will effectively provide high frequency cut-off at any desired frequency and to any desired degree. Filters of the type described above can be manufactured easily and at low cost, and their use, particularly in low priced sound reproducing instruments, can be found very advantageous.

Although I have shown and described several forms of acoustic filter according to my present invention, it will undoubtedly be apparent to those skilled in the art that many other forms thereof, as well as variations in those described, are possible within the spirit of my invention. For example, if desired, the openings in the several plates of the filter need not be arranged in aligned relation. Also, the several plates need not be arranged in parallel relation, nor need the cavities therebetween be of equal volume, although this will be at a sacrifice in the sharpness of cut-off. Other changes will, no doubt, readily suggest themselves to those skilled in the art. I therefore desire that the foregoing description shall be taken as illustrative and not as limiting.

I claim as my invention:

1. A multi-section acoustic filter for attenuating high frequency sounds which comprises at least three similar parallel, perforated plates spaced from each other to provide air spaces between adjacent plates, said plates all having a like number of aligned openings therein, the openings in the end ones of said plates being of one dimension and the openings in the intermediate ones of said plates being of another dimension, said openings of each plate being dimensioned to constitute a non dissipative acoustic impedance, and means connecting said plates to each other around their perimeters to enclose said air spaces.

2. A multi-section acoustic filter for attenuating high frequency sounds which comprises at least three similar parallel, perforated plates spaced from each other to provide air spaces between adjacent plates, said plates all having a like number of aligned openings therein, the openings in the end ones of said plates being of one dimension and the openings in the intermediate ones of said plates being of a smaller dimension than those of the remaining ones of said plates, said openings of each plate being dimensioned to constitute a non-dissipative acoustic impedance, and means connecting said plates to each other around their perimeters to enclose said air spaces.

3. A multi-section acoustic filter for attenuating high frequency sounds which comprises at vleast three similar parallel, perforated plates spaced from each other to provide air spaces between adjacent plates, said plates all having a compared to the remaining ones of said plates that the inertance of the air in each of the openings in said remaining plates is of the order of one-half the inertance of the air in each of said common plate openings, said openings of each plate being dimensioned to constitute a non-dissipative acoustic impedance, and means connecting said plates to each other around their perimeters to enclose said air spaces.

HARRY F. OLSON.

8 References Cited in the tile of this patent UNITED STATES PATENTS Number 1,539,595 1,572,387 1,757,459 1,914,072 1,915,358 2,097,289 2,272,937 2,299,112 2,325,688

15 Number Name Date Powell May 26, 1925 Harrison ..7 Feb. 9, 1926 Harrison May 6, 1930 Boylston June 13, 1938 Giles e June 2'1, 1938 Olson Oct. 26, 1937 Emde Feb. 10, 1942 Schilling e Oct, 20, 1942 Landis Aug. 3, 1943 FOREIGN PATENTS Country Date Germany Sept. 1, 1032 France Oct. 27, 1038 

